Thermo-Mechanical Fatigue Resistant Aluminum Abradable Coating

ABSTRACT

An aluminum coating to be deposited on a substrate having a first coefficient of thermal expansion has an aluminum matrix, and particles of a material having a low coefficient of thermal expansion incorporated into the matrix. The particles bond sufficiently well to the aluminum matrix to carry a portion of the mechanical load.

BACKGROUND

The present disclosure relates to a thermo-mechanical fatigue (TMF) resistant aluminum abradable coating which has particular utility as an outer air seal.

Compressor efficiency is related to blade tip clearance and outer air seal roughness. Coatings for the fan and the low pressure compressor are typically applied on titanium or iron based parts. The coefficient of thermal expansion (CTE) mismatch between the aluminum coating and the base metal (the titanium or iron) is almost a factor of two. This may lead to high compressive stresses in the coating during elevated temperature operation. With many thermal cycles from room temperature and back, the result is thermo-mechanical fatigue cracking and coating spallation.

Blades that mate with an aluminum seal may be titanium- or iron-based. During rub interaction, under certain conditions, metal is transferred to the blade tips. This may cause localized wear of the seal in what is known as record groove patterns. The result is increased average tip clearance and increased roughness in the air flow direction.

It is known in the art to form an aluminum based abradable coating made with hexagonal boron nitride. The problem with this type of coating however is that the hexagonal boron nitride is weak and does not bond well to the matrix.

It is desirable to control the coefficient of thermal expansion of the aluminum coating to be closer to the coefficient of thermal expansion of the base metal and thereby prevent thermo-mechanical fatigue related coating failure.

SUMMARY

In accordance with the present disclosure, there is provided an aluminum coating having a coefficient of thermal expansion which better matches the coefficient of thermal expansion of the substrate to which the aluminum coating is applied.

In accordance with the present disclosure, there is provided an aluminum coating to be deposited on a substrate having a first coefficient of thermal expansion, which aluminum coating broadly comprises an aluminum matrix, particles of a material having a low thermal expansion coefficient incorporated into the matrix; and the particles bonding sufficiently well to the aluminum matrix to carry a portion of the mechanical load.

In another and alternative embodiment, the particles are selected from the group consisting of carbides, borides, oxides, and combinations thereof.

In another and alternative embodiment, the particles are aluminum boride particles.

In another and alternative embodiment, the particles are titanium boride particles.

In another and alternative embodiment, the titanium boride particles have a ratio of titanium to boron in the range of 1:1 to 1:4.

In another and alternative embodiment, the particles are present in an amount from 1.0 to 33 vol %.

In another and alternative embodiment, the particles are present in an amount of from 10 to 25 vol %.

In another and alternative embodiment, the particles are present in an amount of 15 to 20 vol %.

In another and alternative embodiment, the aluminum coating further comprises a pore making material.

In another and alternative embodiment, the pore making material is selected from the group consisting of hexagonal boron nitride, polyester and Lucite.

Further in accordance with the present disclosure, there is provided a process for applying an aluminum coating to a part comprising the steps of: providing a part; forming a powder containing an aluminum matrix and particles having a low thermal coefficient of thermal expansion bonded to the aluminum matrix; and thermally spraying the powder onto the part.

In another and alternative embodiment, the part providing step comprises providing a part formed from a titanium based alloy or an iron based alloy.

In another and alternative embodiment, the powder forming step comprises forming a powder containing particles selected from the group consisting of oxide material, a carbide, a boride, and combinations thereof.

In another and alternative embodiment, the powder forming step comprises forming the powder to have from 1.0 to 33 vol % of the particles.

In another and alternative embodiment, the powder forming step comprises forming the powder to have from 10 to 25 vol % of the particles.

In another and alternative embodiment, the powder forming step comprises forming the powder to have from 15 to 20 vol % of the particles.

In another and alternative embodiment, the powder forming step comprises using titanium boride particles.

In another and alternative embodiment, the powder forming step comprises using aluminum boride particles.

In another and alternative embodiment, the process further comprises adding a pore making material to the powder.

In another and alternative embodiment, the process further comprises melt atomization of a boride containing aluminum alloy and quenching the boride containing aluminum alloy during atomization to form particles of a metastable solid solution.

In another and alternative embodiment, the process further comprises subjecting the aluminum coating to a heat treatment in the range of from 1100 to 1200 degrees Centigrade for 2.0 hours.

Further in accordance with the present disclosure, there is provided a process for forming an aluminum alloy matrix containing fine particles which broadly comprises the steps of: melt atomizing a boride containing aluminum alloy; and quenching the boride containing aluminum alloy during atomization to form particles of a metastable solid solution.

Other details of the thermo-mechanical fatigue resistant aluminum abradable coating are set forth in the following detailed description.

DETAILED DESCRIPTION

As discussed above, there is disclosed an aluminum coating which has a coefficient of thermal expansion which more closely matches the coefficient of thermal expansion of the substrate to which the aluminum coating is applied.

The substrate to which the aluminum coating described herein may be applied may be formed from a titanium alloy or from an iron based alloy. For example, the substrate may be a fan casing or a casing for a compressor section of gas turbine engine.

The aluminum coating has an aluminum matrix formed from an aluminum alloy such as an aluminum-silicon alloy. The aluminum coating further includes fine particles in the range of 20 nm to 5 microns mean particle diameter of a low coefficient of thermal expansion (cte) material, which is defined as having a cte that is lower than that of the aluminum matrix, which are incorporated into the aluminum matrix. In a non-limiting embodiment, the fine particles may be in the range of from 20 nm to 2 microns in mean particle size. The inclusion of the fine particles of a low coefficient of thermal expansion material results in hardening, strengthening and coefficient of thermal expansion reduction. By controlling the amount of fine particles incorporated into the coating, one can control the coefficient of thermal expansion so as to reduce the coefficient of thermal expansion mismatch with the substrate to which the aluminum coating is applied. The amount of fine particles may also be controlled to provide other desirable coating properties.

The fine particles to be incorporated into the aluminum coating may be fine particles selected from the group consisting of an oxide material, a boride, a carbide, and combinations thereof. Only those particles which bond sufficiently well to the aluminum matrix material and are capable of carrying a portion of the mechanical load and which act as a composite material are used in the aluminum coating described herein. For example, the fine particles may be boride particles, such as aluminum boride particles and titanium boride particles.

Aluminum boride particles form high aspect ratio platelets when precipitated at a temperature of below about 650 degrees Centigrade. When aluminum boride particles are used, the technique used to manufacture the aluminum coatings must contain sufficient heating of the alloy constituents to fully dissolve any AlB12 which tends to form at temperatures between 650 degrees Centigrade and 1550 degrees Centigrade depending on boron concentration. The formation of AlB12 during cooling may be suppressed by quenching to below 650 degrees Centigrade. This can bed one by rapid cooling of the melt or passively as one of the characteristics of thermal spray coating in which molten particles are quench cooled upon impact with a surface. If desired, the thermal spray coating may be subsequently heat treated to a temperature of up to 650 degrees Centigrade. Aluminum boride is a desirable candidate due to its low cost, high thermal conductivity, low coefficient of thermal expansion, and good adhesion to the matrix. Aluminum boride at 20 vol % has been shown to increase the strength of aluminum by 80 %.

Titanium boride is also a desirable candidate for the aluminum coating. When using titanium boride, the titanium boride particles may be incorporated into the aluminum matrix by precipitation from a quenched metastable solid solution. The titanium boride may have a ratio of titanium to boron which ranges from 1:1 to 1:4. It is also possible to include the titanium boride particles by mechanical alloying, agglomeration with the matrix alloy in a thermal spray feed stock powder, pressed and sintered.

When borides, such as titanium boride or aluminum boride, are used as the fine particles, they may be present in a range of from 1.0 vol % to 33 vol %, balance aluminum matrix. In another non-limiting embodiment, the boride particles may be present in an amount from 10 to 25 vol %. In still another non-limiting embodiment, the boride particles may be present in an amount from 15 to 20 vol %.

The borides may be dissolved in melt processing, such as melt atomization, to form a spray powder. For example, melt atomization of a boride containing aluminum alloy that quenches during atomization forms particles of a metastable solid solution. This allows deposition of solid or semi-solid particles and subsequent precipitation. The precipitation of undesirable phases may be suppressed as a result of rapid cooling rates. Alternatively, other powder manufacturing routes that result in powder with undesirable phases may be melted and solutionized during the spray coating operation. Either way, molten or softened particles may then be quench cooled during deposition. When an aluminum-titanium-boron alloy is used, titanium boride precipitates from a metastable quenched structure to form precipitates in the 20 nm to 2.0 micron size range, depending on subsequent heat treat temperature. The heat treatment could be heating the aluminum coating at 625 to 650 degrees Centigrade for 2.0 hours.

Carbides which may be used include those in the covalent and interstitial carbide groups with prime candidates being silicon carbide and titanium carbide.

The aluminum matrix with fine particles is formed into a powder with the fine particles is formed into a powder so that it can be applied on the substrate by a spray process such as thermal spraying. The fine particles may be included in the particles forming the aluminum matrix material by agglomeration, alloying and precipitation, or ball milling and cold working. The amount of fine particles is a function of the desired coefficient of thermal expansion for the aluminum coating. Also, considerations of through thickness thermal gradient may be taken into account.

The aluminum coating may be applied to the substrate by thermally spraying the matrix material with the incorporated fine particles. If desired, a pore making fugitive or soft filler material may also be added to the matrix material while it is being sprayed. The fugitive or soft filler material do not affect the coefficient of thermal expansion of the aluminum coating. To affect the coefficient of thermal expansion, the filler material would have to both bond with the aluminum matrix and have sufficient elastic modulus to at least partially elastically constrain the aluminum matrix. The elastic modulus of the filler material may be higher than that of aluminum. Fugitives and fillers like hexagonal boron nitride, polyester, and Lucite are candidates for affecting the abradability of the aluminum coating and may be used to offset the strengthening of the matrix material by the fine particles. Fugitives and soft fillers may be used at up to about 70 volume percent of the coating. In a non-limiting embodiment, the fugitives and soft fillers may be present in an amount from 40 to 65 volume percent. In one example, the coating may be made from an aluminum alloy containing 70.4 wt % aluminum, 13.8 wt % titanium, 9.6 wt % silicon, and 6.2 wt % boron and hexagon boron nitride to produce a coating of 40 volume % metal alloy, 50 volume % hBN and 10% porosity. The titanium boride particles may be precipitated from a quenched metal alloy with heat treatment at 625 to 650 degrees Centigrade for 2.0 hours.

While the aluminum coating particles are sprayed onto the substrate, the sprayed particles may be quenced by conduction of heat to the coating and substrate cooling the particles. After quenching, the substrate with the sprayed particles may be subjected to a heat treatment in the range of from 625 to 650 degrees Centigrade for 2.0 hours.

The benefits of the abradable coating described herein include reduced coefficient of thermal expansion and thermo-mechanical fatigue. This results in a more stable part shape with temperature change and improved coating durability. The filler material also helps to reduce metal transfer to blades during low interaction rate sliding contact wear and result in tighter tip clearance and a smoother coating. As compared to choosing a conventional alloy that has a lower coefficient of thermal expansion, the technique described herein does not increase the incipient melting point of the coating. With regard to the alloy mentioned above, this is because the titanium and boride constituents fully precipitate as a high melting point phase. As a result, the blade tip temperature, particularly of a titanium blade tip, during rub is still limited to the melting point of aluminum. This helps reduce the risk of a titanium fire.

The coatings described herein have good adhesion between the fine particles and the matrix.

There has been described herein a thermo-mechanical fatigue resistant aluminum abradable coating. While the coating has been described in the context of specific embodiments thereof, other unforeseeable alternatives, modifications, and variations may become apparent to those skilled in the art having read the foregoing description. Accordingly, it is intended to embrace those alternatives, modifications, and variations as fall within the broad scope of the appended claims. 

What is claimed is:
 1. An aluminum coating to be deposited on a substrate having a first coefficient of thermal expansion, said aluminum coating comprising: an aluminum matrix; particles of a material having a low coefficient of thermal expansion incorporated into said matrix; and said particles bonding sufficiently well to the aluminum matrix to carry a portion of a mechanical load.
 2. The aluminum coating of claim 1, wherein said particles are selected from the group consisting of carbides, borides, oxides, and combinations thereof.
 3. The aluminum coating of claim 1, wherein said particles are aluminum boride particles.
 4. The aluminum coating of claim 1, wherein said particles are titanium boride particles.
 5. The aluminum coating of claim 4, wherein said titanium boride particles have a ratio of titanium to boron in the range of 1:1 to 1:4.
 6. The aluminum coating of claim 1, wherein said particles are present in an amount from 1.0 to 33 vol %.
 7. The aluminum coating of claim 1, wherein said particles are present in an amount of from 10 to 25 vol %.
 8. The aluminum coating of claim 1, wherein said particles are present in an amount of 15 to 20 vol %.
 9. The aluminum coating of claim 1, further comprising a pore making material.
 10. The aluminum coating of claim 9, wherein said pore making material is selected from the group consisting of hexagonal boron nitride, polyester and Lucite.
 11. A process for applying an aluminum coating to a part comprising the steps of: providing a part; forming a powder containing an aluminum matrix and particles having a low coefficient of thermal expansion bonded to said aluminum matrix; and thermally spraying said powder onto said part.
 12. The process of claim 11, wherein said part providing step comprises providing a part formed from a titanium based alloy or an iron based alloy.
 13. The process of claim 11, wherein said powder forming step comprises forming a powder containing particles selected from the group consisting of oxide material, a carbide, a boride, and combinations thereof.
 14. The process of claim 13, wherein said powder forming step comprises forming said powder to have from 1.0 to 33 vol % of said particles.
 15. The process of claim 13, wherein said powder forming step comprises forming said powder to have from 10 to 25 vol % of said particles.
 16. The process of claim 13, wherein said powder forming step comprises forming said powder to have from 15 to 20 vol % of said particles.
 17. The process of claim 13, wherein said powder forming step comprises using titanium boride particles.
 18. The process of claim 13, wherein said powder forming step comprises using aluminum boride particles.
 19. The process of claim 11, further comprises adding a pore making material to said powder.
 20. The process of claim 11, wherein said powder forming step comprises melt atomization of a boride containing aluminum alloy and quenching the boride containing aluminum alloy during atomization to form particles of a metastable solid solution.
 21. The process of claim 11, further comprising subjecting said aluminum coating to a heat treatment in the range of from 625 to 650 degrees Centigrade for 2.0 hours.
 22. A process for forming an aluminum alloy matrix containing fine particles comprising the steps of: melt atomizing a boride containing aluminum alloy; and quenching the boride containing aluminum alloy during atomization to form particles of a metastable solid solution. 